1. Field of the Invention
The present invention relates, in general, to mobile robots and, more particularly, to a walking robot that performs locomotion or other operations similar to locomotion using movable legs.
2. Description of the Related Art
Recently, research and development on walking type mobile robots, which imitate body mechanisms and motions of some mammals, such as humans and monkeys, performing biped locomotion with an upright posture, have been carried out, so that an expectation of practical use of the walking type mobile robots has increased. Compared with four-legged or six-legged locomotion, the biped locomotion with the upright posture is unstable, and it is difficult to control a posture thereof. However, the biped locomotion is advantageous in that it is capable of coping with a surface having prominences and depressions, such as an uneven surface or an obstacle, and a discontinuous surface, such as stairs or a ladder, so that a flexible mobile operation may be implemented.
A biped walking robot involves many technical difficulties compared with a three or more-legged walking robot in that the biped walking robot should be stably controlled by analyzing mechanisms, dynamics, and system characteristics thereof. Stable locomotion of the biped walking robot may be defined as mobility (locomotion) without tipping over. In particular, stable posture control of the biped walking robot is very important to avoid tipping over of the biped walking robot. The tipping over of the biped walking robot during locomotion implies that an operation being performed is discontinued. If the operation is resumed after the biped walking robot has tipped over and has been set upright, operation time is delayed by at least the discontinued operation time. Further, not only a body of the walking robot suffers from tipping over, but an object colliding with the walking robot may also suffer serious damage by the tipping over of the biped walking robot during locomotion, so that very serious problems may result in some cases. Therefore, stable posture control and prevention from tipping over during locomotion are parts of most important considerations in design and development of the walking robot.
Currently, a locomotion control technique for the biped walking robot employs a Zero Moment Point (ZMP) theory as a criterion to evaluate stability of locomotion. The ZMP theory states that a point where a pitch axis moment and a roll axis moment are zero, that is, a ZMP, exists inside of a ZMP stable region formed by ground contact points of a sole of a foot and a ground surface. According to the ZMP theory, if the ZMP exists inside of a supporting polygon formed by legs and the ground surface, and a force acts in a direction in which the walking robot presses the ground surface at every instant of locomotion, the walking robot does not tip over and performs stable locomotion.
Until now, most of the control algorithms of the biped walking robot have been based on the ZMP theory since the ZMP theory and a numerical formula are relatively simple compared with motion equations of an entire robot, so that a real-time operation is possible. However, to realize dynamic locomotion (using an inertia force) of the biped walking robot by applying the ZMP theory to the biped walking robot, below-described disadvantages among others result.
To apply the ZMP theory to the biped walking robot, a ground reaction sensor, such as a six-axis load cell, should be used on an ankle part, and an acceleration sensor and a gyro sensor should be used on a body or a center of gravity of the biped walking robot. However, the ground reaction sensor is relatively expensive, so that development and manufacturing costs of the biped walking robot increase.
Meanwhile, a solution to a ZMP equation should be obtained, and dynamic locomotion of the biped walking robot should be controlled based on the obtained solution. For this purpose, a high-performance operation processor (that is, a computer) should be used. Furthermore, the dynamic locomotion control of the biped walking robot using the ZMP theory should have exact periodicity, and a Real-Time Operation System (RTOS) should be used for the control. The high-performance operation processor and the RTOS are all expensive, which causes increases in the development and manufacturing costs. In particular, the RTOS incurs many restrictions relating to time when the control of the dynamic locomotion of the biped walking robot is implemented.
Furthermore, to control the biped walking robot using the ZMP theory, data must be accumulated by carrying out many experiments while setting and tuning parameters, and professional knowledge in various fields, such as design, control, dynamics analysis, etc., is required for the carrying out of the experiments. Further, in case of necessity, tools for system analysis, dynamics analysis, and verification of the stability of the control are additionally required. The restrictions described above may decrease a number of opportunities for general public to participate in the development of the biped walking robot, thus acting as a chief factor retarding progress of technical developments of the biped walking robot.